Jump to Previous Strip fails
Solved! Here's what happened:

In order to apply the speed control transform strip, I had imported a new video file to that location on the time line. I had then built a 25% proxy for that file. The video didn't work quite how I wanted, so I deleted the strip and created a new video file with the same name as the deleted one. When I dropped the new file onto the timeline, however, only the audio strip was visible. So I tried deleting the proxy file that had been generated for that file name, and then I imported the new file again. It worked! But the audio strip had actually imported on the previous try: it was hidden behind other strips on the timeline. Clicking nearby strips eventually exposed it, and for some reason that badly imported strip was labeled with a length of 2147483647 frames! Finding and deleting that video strip restored my ability to use the "Jump to Previous Strip" shortcut. Hooray!